In recent years, in response to strong social demands for low fuel consumption of vehicles, weight reduction of tires has been sought. Among tire members, weight reduction and the like of an inner liner also have begun. The inner liner is disposed inwardly in a tire, and reduces an amount of leakage of air from inside to outside the pneumatic tire (decrease in air internal pressure) to improve air permeability resistance.
Currently, a rubber composition for such an inner liner employs, for example, a rubber blend mainly containing a butyl rubber. The rubber blend contains the butyl rubber by 70 mass % to 100 mass %, and a natural rubber by 30 mass % to 0 mass %. In this way, the tire is provided with improved air permeability resistance. In addition to butylene, the rubber blend mainly containing the butyl rubber contains isoprene by approximately 1 mass %, which acts with sulfur, vulcanization accelerator, and zinc white to achieve cross-linking between rubber molecules. In the normal case, the above-described butyl-based rubber needs to have a thickness of 0.6 mm to 1.0 mm for a tire of a passenger car, and needs to have a thickness of approximately 1.0 mm to 2.0 mm for a tire of a truck/bus. In order to achieve weight reduction of such tires, use of polymer, which is more excellent in air permeability resistance and can provide an inner liner layer with a thinner thickness than the butyl-based rubber, has been requested.
Conventionally, in order to achieve weight reduction of a tire, it has been proposed to use a film made of a material including thermoplastic resin, instead of the above-described rubber composition. However, the tire is left outdoor during transportation or display at a dealer, and suffers from deterioration due to ultraviolet radiation of sunlight, so that a thermoplastic elastomer deteriorates to cause a crack, resulting in an impression of bad inner appearance. Moreover, since a pneumatic tire is filled with air in its inner space during use, oxygen in the air will permeate the inside of components constituting the tire, causing oxidation to progress with time. An adverse influence will thus be effected on durability of the pneumatic tire. In particular, if a crack occurs in an inner liner, an impression of bad inner appearance is given to a user. Furthermore, gas barrier property partially deteriorates to decrease tire internal pressure.
In addition, during traveling with the tire, large shear strain acts on a vicinity of a shoulder portion in the inner liner. When the material including the thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
Meanwhile, in order to achieve weight reduction of the inner liner, a technique also has been proposed to employ a thermoplastic elastomer material. However, it is known that a material, which is made thinner in thickness than the inner liner of butyl rubber and exhibits high air permeability resistance, is inferior to the inner liner of butyl rubber in terms of vulcanization adhesion strength with an insulation rubber or a carcass ply rubber adjacent to the inner liner.
When the inner liner has low vulcanization adhesion strength, air enters between the inner liner and the insulation rubber or the carcass rubber, thus resulting in a so-called air-in phenomenon, in which small balloon-like objects appear. The multiplicity of such small spots in the tire gives a user an impression of bad appearance. In addition, during traveling, the air causes detachment to result in cracks in the inner liner. Accordingly, the tire internal pressure is decreased.
In Patent Document 1 (Japanese Patent Laying-Open No. 2009-298986), titanium oxide is blended with a blend of a butyl-based rubber and nylon resin for preventing ultraviolet degradation. However, besides ultraviolet degradation, durability is disadvantageously decreased by deterioration due to radical occurrence in nylon resin caused by flection fatigue.
In Patent Document 2 (WO 2007/116983), a light sealing layer in which carbon black is blended with a mold lubricant for preventing ultraviolet degradation of a thermoplastic elastomer layer is provided on a surface layer. However, due to fluctuations in the step of applying the mold lubricant, the mold lubricant cannot be applied uniformly in the inner surface of the tire, and a scratch may be caused by the hand of an operator in the step or a user, or another reason. Then, the light sealing layer does not achieve its function, resulting in degraded durability due to ultraviolet degradation.